The present invention relates to a method for correcting a vehicle reference speed following a real or apparent deceleration of wheel speed in a vehicle equipped with an anti-lock brake system.
Currently, many passenger vehicles are equipped with anti-lock brake systems (ABS). Commercial vehicles, as well, often utilize such technology, as disclosed for example in "Anti-Blockier-Systeme (ABS) fur Nutzfahrzeuge" (Anti-lock Brake Systems for Commercial Vehicles), published by WABCO, 1991-1992. By adjusting the degree of wheel slippage during hard braking while traveling on a smooth road surface to a predetermined target value, i.e. approximately 15%, optimal braking of the vehicle is achieved, while at the same time the ability to effectively steer the vehicle is maintained.
Regulation of the wheel velocities during braking is based upon regulating signals produced by the electronic system comprising the ABS. These regulating signals generally include wheel acceleration signals (+b), wheel deceleration signals (-b), and wheel slip signals (.lambda.). If one of the wheel velocities (V.sub.R) has been reduced to below a vehicle reference speed (V.sub.Ref), a wheel slip signal (.lambda.) is produced for the particular wheel and, by means of braking pressure release, the wheel is given the opportunity to once again approach the vehicle reference speed (V.sub.Ref). Therefore, in order to insure that the wheel slip signal (.lambda.) is produced at the proper time, the vehicle reference speed (V.sub.Ref) should ideally approximate, as closely as possible, the actual vehicle speed (V.sub.F).
While driving without application of the brake and during periods when the brake is lightly applied, the vehicle reference speed (V.sub.Ref) in a conventional ABS is generally set equal to the highest wheel speed (V.sub.R). However, if during braking of the vehicle it is determined that the deceleration of the wheel speed (V.sub.R) is occurring at more rapid a rate than would be physically possible for that of the entire vehicle, the vehicle reference speed (V.sub.Ref) is no longer equated with the highest wheel speed (V.sub.R), but instead follows a curve, the slope of which is less than that of the measured wheel speed (V.sub.R). Under such conditions, conventional ABS electronic systems adjust the curve which the vehicle reference speed follows to approximately 0.8 g, where g represents ground acceleration.
Although occurring infrequently, there are however conditions under which the vehicle, either actually or apparently, undergoes greater deceleration than the above mentioned 0.8 g. For example, one such instance of a seemingly high rate of vehicle deceleration occurs in an accelerating all-wheel drive vehicle with spinning wheels as a result of a brief failure of the electronic system of the ABS and a subsequent resetting thereof. Following an ABS failure, for example, due to an interruption of the power supply caused by an intermittent contact, the system is automatically restarted. The value given the vehicle reference speed (V.sub.Ref) at the time of restart will then be based upon the spinning wheels, and can therefore be attributed a value much higher than the actual vehicle speed (V.sub.F). As a result, the ABS electronic system interprets the high vehicle reference speed (V.sub.Ref) as an apparent sudden deceleration of the vehicle.
Should the driver of the vehicle attempt to apply the brakes in this operating state, the ABS electronic system incorrectly assumes that the wheels are already rotating too slowly as compared with the erroneously high vehicle reference speed (V.sub.Ref). Consequently, sufficient optimal braking or, in the case of compressed air brakes, any further charging of the brake cylinders, is prevented by operation of the corresponding ABS regulating valves. This effect is present regardless of whether a partial or substantial braking action is initiated. It is therefore possible that optimum brake performance may be adversely affected for a few seconds following the above described system reset. Only after the erroneously excessive vehicle reference speed has returned to a value representative of the actual vehicle speed may the vehicle be decelerated in a normally intended manner.
An apparent high rate of vehicle deceleration may also occur as a consequence of an interference in sensing of wheel speed. The signal voltages produced by wheel speed sensors may be corrupted, for example, by mechanical vibrations and/or electromagnetic interference, such that wheel speed calculated from the sensory measurement far exceeds the actual vehicle speed.
The difference between the measured vehicle reference speed and the actual vehicle speed is interpreted by the electronic system of the ABS as an apparent abrupt deceleration. Although suitable filtering measures are generally taken to suppress such interference in the sensing of wheel speed, the above described false readings still, on occasion, occur.
The same effect can also occur due to an actual uneven road surface. In this case also, in spite of correct functioning wheel speed sensors, a too high vehicle reference speed can occur.
The above effect is not limited merely to apparent vehicle deceleration, but can also occur in the case of an actual, sudden vehicle deceleration due to an accident.
To solve the above stated problem, a separate vehicle deceleration sensor which would detect the actual vehicle deceleration and transmit the information to the electronic system of the ABS for correction of the erroneously high vehicle reference speed has been suggested. The high cost associated with such approach is, however, a disadvantage.
It is therefore an object of the invention to provide a method which shortens the duration of reduced brake performance due to actual or apparent abrupt vehicle deceleration by initiating a more rapid restoration of proper regulating capability of the ABS in an economical manner.